This invention relates to superplastic forming and diffusion bonding and more particularly to a pressurized system for superplastic forming and diffusion bonding a sandwich-like construction made of a plurality of layers of metal such as titanium.
The process of superplastic forming and diffusion bonding is well known in the art. The process has the advantages of forming complex shapes and permitting deep drawing of metallic parts. At the temperatures involved the deformation stresses are relatively low, which permits the forming of complex parts under pressures which keep tool deformation and wear at a minimum.
Diffusion bonding which is used in conjunction with superplastic forming refers to the metallurgical bonding or joining of metal by the steps of applying heat and pressure for periods of time sufficient to cause co-mingling of the metal atoms at the places where the surfaces are joined.
The superplastic forming and diffusion bonding process is particularly useful in the aerospace industry in the formation of wings, frames and the like. The process is particularly useful in forming strengthening members used in aerospace structures referred to as frames and stringers.
Generally speaking, the process begins with cleaning the metal sheets to be used in the sandwich. The sheets are then selectively coated with a material commonly referred to as "stop-off" which typically is boron-nitride, yttria or some rare earth or other very inert material. The purpose of the "stop-off" is to prevent diffusion bonding of the sandwich-like material where the stop-off material has been applied. The areas coated with "stop-off" can be formed into a desired shape by the application of gas pressure applied to the inside of the metallic sandwich. Prior art systems have not been reliable for commercial production. In some prior art practices, an inlet tube was inserted between the sandwich layers to be used to evacuate the space between the sandwich layers and to thereafter supply gas under pressure to the spaces between the sandwich to form the sandwich into the desired honeycomb shape by superplastic forming.
In prior practices, after inserting the tube between the sandwich layers, the sandwich construction sometimes was welded around its periphery to form a unitary structure or was bonded during the superplastic forming, diffusion bonding cycle to form an assembly sometimes called a bag. Typically, this was accomplished by seam welding or under heat and pressure by diffusion bonding. In many operations there were more than one such inlet tubes.
One problem that has arisen as a result of these prior art practices where the inlet tube extended into the sandwich was that while welding the peripheries of the sandwiched sheets one or more of the inlet tubes would be pinched closed and rendered inoperable. Where this occurred, the subsequent operations were severely adversely affected, sometimes resulting in complete failure to form the desired assembly.
In another approach a tube was welded to the bag. The weld is susceptible to cracks that could break the seal which should exist between the inlet tube and the sandwich construction which also would adversely affect the superplastic forming operation.
There is a need, therefore, for a system for superplastically forming and diffusion bonding which is reliable, simple, inexpensive and yet adapted for high production use.